Teacher background information

Many chemical reactions require the input of energy to initiate them. In cases, such as combustion, the exothermic nature of the reactions themselves provides sufficient energy to sustain the ongoing reaction. Fire, the result of a combustion reaction, is important in ecosystems, such as the tropical savanna regions of northern Australia, as it promotes the recycling of nutrients. This process is well-known by Aboriginal and Torres Strait Islander peoples and as such, fire has been used for millennia to control the transfer of matter and energy through the ecosystem in a practice known as firestick farming. 

The Sun’s energy is captured and trapped in plant material through the process of photosynthesis. This energy is primarily utilised through the process of respiration and as it is transferred through food webs it sustains all life in an ecosystem. During fires, the energy stored in biomass is released through the process of combustion. The process of burning biomass releases a wide range of products, some of which can act as nutrients or contribute to the cycling of nutrients in the environment. Others, such as methane and dioxins, are harmful to the environment. Which products are produced, and in what quantities, can be significantly affected by the temperature at which the biomass is burnt. Studies have found that the temperature of the fire also has an impact on the physical, chemical and biological properties of soils.   

Over millennia, Aboriginal and Torres Strait Islander peoples have amassed knowledge about the variables that affect the control and intensity of fires. This knowledge is applied to regulate the frequency and timing of fires in order to ensure the efficient recycling of nutrients, thus creating and shaping a patchwork of resources at different stages of regrowth in a convenient and predictable manner. Fires that are burning too hot result in a greater loss of volatile nutrients from the ecosystem and have detrimental effects on the soil. Aboriginal and Torres Strait Islander peoples understand this fact well. Hence, burns are conducted in the early dry season, whilst plant matter still contains moisture, resulting in smaller scale and cooler fires. This method of harnessing fire in strictly controlled fire regimes is known as firestick farming and is the oldest known farming practice in the world.  

The successful transfer of nutrients is highly dependent on fire temperature. An essential factor in controlling fire temperature is the time when these fires occur. In recognition of this, Aboriginal and Torres Strait Islander communities in the tropical savanna areas of northern Australia purposefully light fires in the early dry season (March-April) to prevent the occurrence of hotter burning fires in the late dry season (October-November). These low-intensity fires recycle nutrients from the biomass into new growth, allow for the selective distribution of fire-tolerant and fire-sensitive plants, foster the regrowth of crop plant species, and encourage the migration of target game species to the new regrowth zones of the recently burnt areas. At the same time, the mosaic nature of burnt and unburnt areas provides a balanced environment for game species to find food and shelter. 

Students could research the chemical reactions of photosynthesis, respiration, the complete combustion of simple hydrocarbons in an ample supply of oxygen and the incomplete combustion of simple hydrocarbons. They could compare the products of these combustion reactions with those produced in the combustion of the more complex compounds involved in biomass fuels, and the effect of high temperature fires on the production of greenhouse gases such as methane and nitrogen oxides. 

In investigating the context suggested in this elaboration students will gain a deeper understanding of the role combustion reactions play in the accessibility and recycling of nutrients through the ecosystem and how these reactions have been employed and controlled by Aboriginal and Torres Strait Islander peoples in the agricultural practice of firestick farming.